1. Field of the Invention
This invention relates to a fixing device for use in an image forming apparatus such as a copying apparatus or a printer.
2. Related Background Art
FIG. 3 of the accompanying drawings shows an example of an image forming apparatus. When the image forming operation is started in this apparatus, a signal is first sent to a laser transmitter by a controller (not shown) and a laser beam is emitted. This laser beam is then reflected by a mirror 9 (generally called a polygon mirror) which is rotating, and is further turned back by a reflecting mirror 10, whereby the laser beam is applied onto a photosensitive drum 12.
By such a series of operations, a so-called latent image is formed on the photosensitive drum 12, and this latent image is developed by a developing apparatus 11, whereafter the developed image is transferred as a toner image onto a recording medium.
Sheets of paper as recording mediums are supplied one by one from a paper cassette by a pickup roller 13 and pass under the photosensitive drum 12. A toner image is transferred onto a sheet of paper by the aforedescribed process as the sheet of paper passes. Thereafter, the sheet of paper is directed to a pair of fixing rollers 1 and 2, is urged by the pair of fixing rollers and is heated by the heat of heaters provided in the rollers whereby the toner image is fixed on the sheet of paper. An upper fixing separating pawl 3 and a lower fixing separating pawl 4 are in contact with the fixing rollers 1 and 2, respectively, whereby the sheet of paper is separated from the fixing rollers 1 and 2. The separated sheet of paper is conveyed to the outside of the apparatus body by a pair of paper discharge rollers 5 and 6, thus terminating the image forming process.
The upper fixing separating pawl 3 and the lower fixing separating pawl 4 adopt a construction as shown in (a) of FIG. 4 of the accompanying drawings. That is, the separating pawls 3 and 4 are pivotally movable about centers of rotation 3a and 4a, respectively, and are urged against the rollers by tension springs 13 and 14, respectively. Accordingly, when the leading end of the sheet of paper is jammed by some cause or other, the sheet of paper continues to be conveyed until the jamming of the paper is detected by paper detecting means (not shown) on the downstream side and the driving of the body is stopped. Therefore, the sheet of paper is jammed into an accordion-like shape between an upper paper discharge guide 7 and a lower paper discharge guide 8. At that time, as shown in (b) of FIG. 4 of the accompanying drawings, the sheet of paper jammed into the accordion-like shape pushes up the rear end portion of the upper fixing separating pawl 3 and therefore, the separating pawl is rotated and the tip end of the separating pawl becomes separated from the fixing roller 1. As the process further proceeds to (c) and (d) of FIG. 4 of the accompanying drawings, the sheet of paper is forced into a gap formed between the tip end of the separating pawl 3 and the fixing roller 1, and finally, as shown in (e) of FIG. 4 of the accompanying drawings, the sheet of paper jammed into the accordion-like shape will break the tip end of the separating pawl.
Usually, the tip end of the separating pawl is sharply formed into an "r" shape of several tens of .mu.m and is weak in strength. This may lead to the possibility that the paper will pierce the tip end to thereby damage the shape of the tip end. There has also been a problem that the paper pierces the tip end of the separating pawl to thereby make it difficult to eliminate the jammed paper. A similar problem arises with regard also to the lower fixing separating pawl 4. A similar problem also arises in a drum separating pawl which is in contact with the photosensitive drum to separate the paper from the photosensitive drum.
In order to solve such problems, it would be possible to adopt a construction as shown in FIG. 5 of the accompanying drawings. That is, the rear end portions of the upper separating pawl 3 and lower separating pawl 4 are constructed so as to be covered with the portions 7a and 8a of the upper paper discharge guide 7 and lower paper discharge guide 8, respectively.
By the adoption of such a construction, even if the paper is jammed into an accordion-like shape when it just leaves the pair of fixing rollers 1 and 2, it will become difficult for the paper to push the rear ends of the separating pawls and therefore, it will become difficult for the phenomenon as noted above to occur.
However, even if such a construction is adopted, there are some gaps for permitting the pivotal movement of the separating pawls between the upper paper discharge guide 7 and the separating pawl 3 and between the lower paper discharge guide 8 and the separating pawl 4, and if the paper comes into these gaps, the rear ends of the separating pawls 3 and 4 will be pushed in the same manner as described above and the tip end of the separating pawl will become separated from the fixing roller 1.
Also, the guide plates 7 and 8 usually formed by metal sheets are inferior in slidability relative to the paper P, as compared with the surfaces of the separating pawls, and this gives rise to the problem that the guide plates scrape off the image on the surface of the paper P to thereby damage the image. Further, it is generally known that when the paper P is heated by the fixing roller 1, the moisture in the paper evaporates and is condensed on the upper and lower paper discharge guides 7 and 8 formed by metal sheets, and by the guide plates adopting the shapes as shown in FIG. 5, waterdrops created by the condensation become ready to adhere to the paper. The waterdrops adhering to the paper remarkably hamper the conveyance of the paper conveyed between the guide plates and therefore cause the paper to be jammed.